How To Build Your Own Radius Gauges

This is the Project we're going to build - DIY Radius Gauges

In recent years, the ubiquity of online information and specialized luthiery retailers has significantly reduced the barrier to entry for those who want to start building guitars. It is easier than ever to buy luthier-specific tools that people had to make for themselves not that long ago. While it may be easy to buy radius gauges from luthier suppliers, making your own can be a way for you to learn things about your tools and your own creative abilities. You would not be able to learn from just clicking on something on your favorite website and having it arrive in the mail a few days later. Here’s a short tutorial on how to make yourself some radius gauges so you can practice using your tools and problem solving skills, while at the same time making something useful for your shop.

Materials Needed:

In order to make these guages, you will need the following materials:

1. Some Kind of 1/8 inch thick material. I used some tempered hardboard that I had that was left over from a Danelectro-style guitar build I am doing. It looks like this:

2. You’ll also need some kind of string, yarn or twine, and you’ll need roughly 24 inches of it. In my case, I used butcher’s twine
that I keep on hand for wrapping vintage-style Telecaster bridge pickups. It looks like this:

1. A 24 inch (or longer) Straight Edge

2. Bandsaw or Coping Saw

If you’re tight on money or space, but have lots of time, the coping saw may be your only option. The learning curve with hand tools is fairly steep, however, so be prepared to make a lot of scrap while you’re learning to cut close to a pencil line with the coping saw. If you need a coping saw, here’s a link to a good one to start with:

4. Brad Point Drill Bit Set

While not absolutely critical, I like to drill holes in my tools and fixtures for easy hanging in the shop. I like Brad Point bits because they are easy to keep level. Here’s a decent set to get you started:

5. Polyurethane Finish

It’s a good idea to finish the tools, jigs and fixtures you make out of plywood, MDF or tempered hardboard with some kind of hard finish to protect them from fluctuations in humidity. Here’s the stuff I use:

Making The Gauges

Here is the method I used to make a set of two general purpose radius gauges. The plan is to make two radius gauges, one with the radii 7.25 inches, 9.5 inches, 10 inches, and 12 inches; and the other with the radii 14 inches, 15 inches, 16 inches and 20 inches. We will end up with two gauges that resemble ninja stars.

Step One: Make The Pivot Point

Since a radius is defined as the length from the center of a circle to its edge, we need a point to which we will tie our twine.

In my case, I used a regular drywall screw screwed directly into my work bench, but you could use a nail. Like so:

Drywall Screw as a Pivot Point

Step Two: Tie The Twine To The Pivot Point

Next, we need to tie one side of the twine to the pivot point, like so:

Tie one end of the twine to the screw

Step Three: Lay The Straight Edge On The Work Surface

We need to have an accurate measurement from the center of the pivot point to the radius we desire, so make sure the straightedge is measuring from the CENTER of the screw.

Step Four: Attach The Marking Implement, And Roll It To The Correct Radius

Here’s how I do this: I simply wrap my pencil in the twine until the point of the pencil is at the correct radius while the length of twine from the screw to the pencil remains taut. It is extremely important that you keep the marking instrument perfectly vertical.

Measure the radius with a ruler

Step Five: Make A Radius Mark On Your 1/8th Inch Material

Next, simply slide your 1/8th inch material under the path of the marking instrument and make a mark. Take care to keep the instrument vertical as you are making the mark. Make sure you label the length of the radius to the inside of the mark, it will save you huge headaches later.

Scribing the radius on the hardboard

Step Six: Rotate the 1/8th Inch Material 90 degrees

Next, rotate your material 90 degrees, so you can make a new mark.

Step Seven: Repeat Steps 4, 5, and Six Three Times

After you get the hang of it, you can get into measuring and marking fairly quickly. You should end up with something that makes sort of a diamond pattern.

Measuring for 12 inch Radius radius

Marking the 12 inch radius on the hardboard

Measuring 10 inch Radius

Marking the 10 inch Radius on the hardboard

This is the final radius mark for first gauge

Step Eight: Repeat Steps 4 Through 7 On A New Piece Of 1/8th Inch Material

Once you have one gauge marked, get yourself a new piece of 1/8th inch material, and go through the same steps with the remaining radii.

Step Nine: Roughly Cut Radius Marks With A Band Saw or Coping Saw

Take your marked pieces to your band saw or use a vice and a coping saw and cut close to the marks without touching them. The more accurately you can cut, the easier the sanding with be later.

Rough Cutting Gauge 1

Rough Cutting Gauge 2

Both Gauges Rough Cut

Step Ten: Using A Spindle Sander, Drill Press, or Cordless Drill, Sand Up To The Mark

I used my drill press with a drum sanding attachment to approximate a spindle sander. Just patiently creep up to the line while you’re sanding–don’t get too aggressive and go through the line.

Sanding Drum attachment for drill press

Sanding Drum On Drill Press

Sanding The Edges

Step Eleven: Remove The Sharp Corners

I used the band saw to cut the menacing corners off the gauges, then I lightly rounded them over with a file.

Cutting the sharp corners

Smoothing out the corners with a file

Step Twelve: Label The Radii With A Permanent Marker

I felt that the pencil I used to label the radii initially wasn’t dark enough for me to see clearly, so I labeled them again with a permanent marker so I would be able to see them.

Labelled the radii with a permanent marker

Step Thirteen: Drill Hanging Holes In The Gauges

I don’t have much tool storage space in my shop, so I have to hang my tools and fixtures.

Drilling a hole on gauge 1

Drilling a hole on gauge 2

Step Fourteen: Brush On Moisture Protection.

I like to keep my templates, jigs and fixtures safer from humidity by applying a coat of clear polyurethane before use. Just let them dry at least 24 hours before trying to use them on an instrument.

Applying Finish

Finished Radius Gauges

Radius gauge project in use

Conclusion

Radius gauges are an essential tool that every luthier –hobbyist or pro– needs in his or her shop. Sure you could buy some, but then you wouldn’t get the satisfaction of knowing that you built some of your own equipment. You even learned more about your tools and your own resourcefulness in the process. Use these experiences to inform the decisions you make while building jigs templates and guitars in the future, and have fun!